Why rationalism, you ask?
I argued earlier that everything happens mechanistically, nothing magically. My argument for that conclusion assumed that the universe is intelligible: that is, an infinite intellect could make complete sense of it. But an infinite intellect could make complete sense of the universe only if rationalism is true ‒ only if, for each true proposition p, there’s a reason that p is true. Otherwise, an infinite intellect would encounter some true proposition without being able to understand why it’s true.
“Still,” you might ask, “why is rationalism true in the first place?” One way of replying is to give a “transcendental” argument. According to such an argument, you presume the truth of rationalism in the very act of asking why rationalism is true. When you ask why rationalism is true, you expect an answer other than “It just is.” Indeed, if I replied to your question with “Why do you ask?” you would make your expectation explicit: “Because if rationalism is true, then there’s a reason it’s true.”
Although the conditional statement “If rationalism is true, then there’s a reason it’s true” is a tautology, you don’t assert it as a tautology. Instead, your conditional statement functions as a substantive assertion. I say so because you also think that rationalism couldn’t be false for no reason. You think, “If rationalism is false, then there’s a reason it’s false,” which isn’t a tautology.
Because you presume that rationalism, if true, is true for some reason, you have no principled basis not to presume the same thing with regard to every truth there is. In particular, there’s no basis in the claim that explanation must eventually end with brute facts, for that claim is false. But if every truth is true for some reason, that’s rationalism by definition.
This argument doesn’t conclude that rationalism is true, only that you presume its truth in asking why it’s true. Can a more ambitious argument show that rationalism is true? I leave that question for a future post.
